Seeing Below Ground Without Moving Dirt

Geophysics 101

Throughout the Time Team America digs you will see people walking along steep and rocky ground, maneuvering through a maze of ropes, pin flags, and rattlesnakes, often with ridiculously bulky things strapped to them. They are the brave geophysicists, Meg, Bryan and Duncan, and they are engaging in some serious science!

The geophysics team is a crucial aspect of the Time Team digs—with only three days to investigate each site, geophysics gives a first look at what might lie under the soil. This provides much-needed clues about where best to focus our archaeological efforts. When confronted with a large area and little time, the prospect of finding a single building or small feature can be daunting. Using Meg and Bryan's expertise and geophysical survey maps, we quickly established areas most likely to hold archaeological resources, and put the diggers to work.

Geophysical technology is an increasingly important aspect of modern-day archaeology. It allows us to take a peek at what lies buried beneath the soil, without ever disturbing it. fs_gpr.jpg Archaeology, by its very nature, is destructive, and by digging a site you are in fact destroying it. It's helpful to have non-intrusive technologies that can gather data while leaving the site virtually intact for future research. Geophysics can even be used in order to avoid archaeological sites during construction projects like road work or pipeline installations.

Archaeology is an expensive endeavor. Gathering geophysical data before an excavation can save time and money by focusing resources on the high-probability areas. At Fort James, geophysical maps saved hours of excavation by providing Time Team America a visual outline of the buried Fort ruins. This allowed targeted areas to be examined and compared with the original construction plans of the Fort. The more information archaeologists have before they put the shovel to the soil, the more data each scoop will deliver.

Method to the Madness
Geophysicists gather data by surveying along established grid lines, which allows them to make an "underground map" of the site. It is important to create a reliable site grid. Geophysical maps need to be able to accurately reference specific areas of the site in order for the results to be useful. Survey data is first processed on a computer, and then the blobs and squiggles are carefully analyzed by the highly-trained geophysicists. As Season 1 Host Colin put it, Meg reads geophysics maps "like a radiologist reads an X-ray." What she is looking for are anomalies, something that is different or stands out from everything around it. Anomalies represent changes in the soil that could mean a buried feature such as a hearth, abandoned well or garbage midden—archaeological paydirt!

Depending on environmental conditions and the research goals of the site, geophysicists have a variety of  technologies upon which to draw. The three most commonly used in archaeology are Magnetometry, Resistivity, and Ground Penetrating Radar. These technologies can be used alone or in conjunction to add levels of detail to the maps.

Too Good to be True?
While geophysical data is a great tool, it's only one part of the archaeological process. Geophysics rarely provides a perfect picture. Certain soil conditions can make it hard to "see" under the ground. Tall grass or difficult walking conditions can make it hard to create a reliable Lawton_Dig.jpg map. Features that might look like walls or garbage pits in the data may turn out to be natural rock or soil formations that have never been altered by humans. The only way to know for sure is to start digging. By excavating, or "ground-truthing," a hot spot on a map, archaeologists gather physical evidence to confirm the geophysical results. Sometimes this leads to great discoveries; other times it is a dead end. But even negative results contain valuable data. Excavating a surveyed area can provide insight into local soil conditions and help explain what the geophysics is seeing. This provides researchers a better understanding of the geology of a site, the people who lived there, and the soil formation processes that have taken place since it was abandoned.

Geophysical technology is continually evolving and improving. The more geophysics is used on archaeological sites, the better it can be used to anticipate and interpret finds. Meg and Bryan continue to experiment with new gadgets and gizmos, and have pioneered new uses of geophysical technology. Mapping the underground quarry debris at the Topper site, for example, was a huge success, and shows promise for exciting new innovations in geophysical archaeology.

MORE GEOPHYSICS

Ground-penetrating Radar

Ground-penetrating Radar

GPR maps features such as walls, trenches, pits, and more.

Magnetometry

Magnetometry

Magnetic survey measures the variation of magnetic fields of the earth.

Resistivity

Resistivity

Resistance survey measures the subsurface variation in soil.

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